In a makefile, how to get the relative path from one absolute path to another?

Question

An example to illustrate my question:

Top level makefile

rootdir = $(realpath .)
export includedir = $(rootdir)/include
default:
    @$(MAKE) --directory         


        

Answer 1

Robust drop-in solution with pure Make:

override define \s :=
$() $()
endef

ifndef $(\s)
override $(\s) :=
else
$(error Defined special variable '$(\s)': reserved for internal use)
endif

override define dirname
$(patsubst %/,%,$(dir $(patsubst %/,%,$1)))
endef

override define prefix_1
$(if $(or $\
$(patsubst $(abspath $3)%,,$(abspath $1)),$\
$(patsubst $(abspath $3)%,,$(abspath $2))),$\
$(strip $(call prefix_1,$1,$2,$(call dirname,$3))),$\
$(strip $(abspath $3)))
endef

override define prefix
$(call prefix_1,$1,$2,$1)
endef

override define relpath_1
$(patsubst /%,%,$(subst $(\s),/,$(patsubst %,..,$(subst /,$(\s),$\
$(patsubst $3%,%,$(abspath $2)))))$\
$(patsubst $3%,%,$(abspath $1)))
endef

override define relpath
$(call relpath_1,$1,$2,$(call prefix,$1,$2))
endef

Test cases:

$(info $(call prefix,/home/user,/home/user))
$(info $(call prefix,/home/user,/home/user/))
$(info $(call prefix,/home/user/,/home/user))
$(info $(call prefix,/home/user/,/home/user/))

$(info $(call relpath,/home/user,/home/user))
$(info $(call relpath,/home/user,/home/user/))
$(info $(call relpath,/home/user/,/home/user))
$(info $(call relpath,/home/user/,/home/user/))

$(info ----------------------------------------------------------------------)

$(info $(call prefix,/home/user,/home/user/.local/share))
$(info $(call prefix,/home/user,/home/user/.local/share/))
$(info $(call prefix,/home/user/,/home/user/.local/share))
$(info $(call prefix,/home/user/,/home/user/.local/share/))

$(info $(call relpath,/home/user,/home/user/.local/share))
$(info $(call relpath,/home/user,/home/user/.local/share/))
$(info $(call relpath,/home/user/,/home/user/.local/share))
$(info $(call relpath,/home/user/,/home/user/.local/share/))

$(info ----------------------------------------------------------------------)

$(info $(call prefix,/home/user/.config,/home/user/.local/share))
$(info $(call prefix,/home/user/.config,/home/user/.local/share/))
$(info $(call prefix,/home/user/.config/,/home/user/.local/share))
$(info $(call prefix,/home/user/.config/,/home/user/.local/share/))

$(info $(call relpath,/home/user/.config,/home/user/.local/share))
$(info $(call relpath,/home/user/.config,/home/user/.local/share/))
$(info $(call relpath,/home/user/.config/,/home/user/.local/share))
$(info $(call relpath,/home/user/.config/,/home/user/.local/share/))

$(info ----------------------------------------------------------------------)

$(info $(call prefix,/home/user/.local/share,/home/user))
$(info $(call prefix,/home/user/.local/share,/home/user/))
$(info $(call prefix,/home/user/.local/share/,/home/user))
$(info $(call prefix,/home/user/.local/share/,/home/user/))

$(info $(call relpath,/home/user/.local/share,/home/user))
$(info $(call relpath,/home/user/.local/share,/home/user/))
$(info $(call relpath,/home/user/.local/share/,/home/user))
$(info $(call relpath,/home/user/.local/share/,/home/user/))

$(info ----------------------------------------------------------------------)

$(info $(call prefix,/home/user/.local/share,/home/user/.config))
$(info $(call prefix,/home/user/.local/share,/home/user/.config/))
$(info $(call prefix,/home/user/.local/share/,/home/user/.config))
$(info $(call prefix,/home/user/.local/share/,/home/user/.config/))

$(info $(call relpath,/home/user/.local/share,/home/user/.config))
$(info $(call relpath,/home/user/.local/share,/home/user/.config/))
$(info $(call relpath,/home/user/.local/share/,/home/user/.config))
$(info $(call relpath,/home/user/.local/share/,/home/user/.config/))

$(info ----------------------------------------------------------------------)

$(info $(call prefix,/root,/home/user))
$(info $(call prefix,/root,/home/user/))
$(info $(call prefix,/root/,/home/user))
$(info $(call prefix,/root/,/home/user/))

$(info $(call relpath,/root,/home/user))
$(info $(call relpath,/root,/home/user/))
$(info $(call relpath,/root/,/home/user))
$(info $(call relpath,/root/,/home/user/))

Expected results:

/home/user
/home/user
/home/user
/home/user




----------------------------------------------------------------------
/home/user
/home/user
/home/user
/home/user
../..
../..
../..
../..
----------------------------------------------------------------------
/home/user
/home/user
/home/user
/home/user
../../.config
../../.config
../../.config
../../.config
----------------------------------------------------------------------
/home/user
/home/user
/home/user
/home/user
.local/share
.local/share
.local/share
.local/share
----------------------------------------------------------------------
/home/user
/home/user
/home/user
/home/user
../.local/share
../.local/share
../.local/share
../.local/share
----------------------------------------------------------------------




../../root
../../root
../../root
../../root